Key Sulfur-Containing Compounds in the Atmosphere and Ocean

Chapter 25 Key Sulfur-Containing Compounds in the Atmosphere and Ocean Determination by Gas Chromatography—Mass Spectrometry and Isotopically Labeled Internal Standards

Downloaded by PENNSYLVANIA STATE UNIV on August 25, 2013 | http://pubs.acs.org Publication Date: September 8, 1992 | doi: 10.1021/bk-1992-0502.ch025

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Alan R. Bandy, Donald C. Thornton, Robert G. Ridgeway, Jr. , and Byron W. Blomquist Chemistry Department, Drexel University, Philadelphia, PA 19104

Gas chromatographic/mass spectrometric (GC/MS) methods using isotopically labeled internal standards (GC/MS/ILS) are described for determining atmospheric sulfur dioxide (SO ), dimethyl sulfide (DMS), carbon disulfide(CS ),dimethyl sulfoxide (DMSO), dimethyl sulfone(DMSO )and carbonyl sulfide (OCS) and aqueous phase dimethyl sulfide and dimethyl sulfoxide. GC/MS/ILS has great immunity to variations in sampling efficiency and changes in detector sensitivity. Using cryogenic preconcentration and integrationtimesof three minutes, lower limits of detection are below one part per trillion for these gas phase species. Lower limits of detection for aqueous phase measurements are better than one picomole. Measurement precision is limited by either the lower limit of detection or the repeatability of the addition of the standard. Accuracy is determined primarily by the accuracy of the standards. GC/MS/ILS appears to have the sensitivity and precision to make real time isotopic ratio measurements. 2

2

2

Isotopic dilution methods are widely employed for improving the reliability of difficult determinations (1). Despite their advantages, however, they are rarely used in atmospheric and oceanic science. We describe in this paper isotopic dilution methods for several key sulfur species present in the atmosphere and ocean. We use a variation of isotope dilution in which isotopically labeled analyte is added to the sample as an internal standard. Only stable isotopes are used. Analyses are carried out by GC/MS. Current address: Air Products and Chemicals, Inc., Allentown, PA 18195

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0097-6156/92/0502-0409$06.00/0 © 1992 American Chemical Society

In Isotope Effects in Gas-Phase Chemistry; Kaye, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

410

ISOTOPE EFFECTS IN GAS-PHASE CHEMISTRY

Advantages of G C / M S / I L S Compared to other methods the isotopic dilution GC/MS method, GC/MS/ILS, has several important advantages: Insensitivity to sampling losses Insensitivity to changes in GC/MS sensitivity


Isotopically labeled standard trapped with each sample Large linear dynamic range High Sensitivity - low detectable limits High manifold analyte concentration due to labeled standard Insensitivity to sampling losses and changes in GC/MS sensitivity are demonstrated by considering two isotopomers simultaneously monitored. Instrument responses for the two isotopomers are given by the expressions: H

=

\

a

C

i*i i

H = a ib,C 2

2

W

2

Here H, a, k and C are the instrument response, sampling efficiency, instrument sensitivity and analyte concentration respectively. If isotopomer 1 is in ambient air only and isotopomer 2 in the standard only, the ambient air concentration is given by the expression

H

2

We have assumed that the sampling efficiencies and the instrument responses are the same for the two isotopomers. For S labeled isotopomers we found no differences in sampling efficiencies for standard and ambient isotopomers. For deuterated standards we observed no differences in sampling efficiencies or instrument sensitivities for the ambient and standard isotopomers, however, the retention time of DMS-c^ was less than the retention time of DMS by a few seconds. The retention time was shorter for DMS-ί^ than for DMS apparently because DMS forms stronger hydrogen bonds to the stationary phase than DMS-cV The retention time difference was observable because of the large number of equilibrium steps (theoretical plates) in the separation which is very sensitive to small differences in interaction with the stationary phase. M



25. BANDY ET AL.

Key Sulfur-Containing Compounds

411

Our studies confirm that the calculated ambient concentration is independent of the instrument sensitivity and the sampling efficiency as the above argument suggests. A slightly more complex equation presented in a subsequent section applies when isotopomers 1 and 2 are in both ambient air and the standard. As in the simple case the sampling efficiency and instrument sensitivity are absent from the final expression in this more complex case. An internal calibration is included in every sample. Since the internal standard is an isotopomer of the analyte it has the same chemistry as the analyte, at least within the measurement precision of the common GC/MS systems. Inclusion of the isotopically labeled internal standard eliminates the effect of the sample matrix on precision and accuracy. Estimates of sampling efficiencies and instrument calibrations using test atmospheres prepared in "zero grade" air ignore the matrix effect on calibration. This approach often fails in atmospheric sampling except for the most inert atmospheric constituents. Standard addition calibrations are a partial solution. In standard addition calibrations using nonisotopically labeled standards, instrument response is assumed to be proportional to analyte concentration over the entire dynamic range of the analyte. Furthermore, the proportionality coefficient is assumed to be constant. For reactive atmospheric species these conditions may not be met, especially at low analyte concentrations. Standard addition calibration using isotopically labeled standards circumvents all of these problems. Accurate calculations of the lower limit of detection and sensitivity for every sample and no need of a special calibration sequence that would decrease the sampling rate are also important advantages. The mass spectrometer, MS, is an extremely sensitive and specific detector. The lower limit of detection is about 4 femtomoles per second, which is at least 10 times better than the flame photometric detector and very close to the lower limit of detection of the electron capture detector. High sensitivity and the collection of large samples make measurements possible even in high loss conditions. Analyte losses are typically less than a few percent for gas phase sulfur dioxide, S 0 , carbonyl sulfide, OCS, and carbon disulfide, CS . Because of oxidation in the trap, losses are higher and more variable for gas phase dimethyl sulfide, DMS, dimethyl sulfoxide, DMSO, and dimethyl sulfone, D M S 0 . Oxidant scrubbers installed in the manifold just after the air driers and before the cryogenic trap reduce these losses to manageable levels. Variable trapping and conversion efficiencies exist in the determinations of aqueous DMS, DMSO and D M S 0 . The isotopically labeled internal standard, however, makes the GC/MS/ELS immune to these types of the potential errors. Because the standard concentration in the manifold is maintained at about 500 pptv, the manifold always contains a high concentration of analyte. To illustrate the advantage, consider a manifold that will remove 10 pptv. If no internal standard is present and the ambient concentration is 20 pptv, the instrument will yield a result of 10 pptv. If 500 pptv of isotopically labeled standard analyte is present in the manifold the total manifold concentration will be 520 pptv. As in the previous example 10 pptv will be removed by the manifold 2

2

2

2



412


and the instrument will yield 510 pptv. However, the standard and ambient air analyte will be lost in the same proportion, therefore, the instrument will yield (510/520)*20 = 19.6 pptv for ambient air representing only a 2 percent error. This result should be compared to the 100 percent error that results if the manifold contains no standard analyte. High concentrations of analyte in the manifold also decrease the manifold equilibration time. Adsorptive sites can cause long equilibration times, especially at low concentrations where an appreciable portion of the analyte is adsorbed in passing through the measurement system. For Teflon manifolds connected with machined stainless steel fittings, there are relatively small numbers of these sites. In the GC/MS/ILS manifold such sites are occupied primarily by isotopically labeled analytes, which are typically in large excess in the manifold. Therefore, fluctuations in the ambient air concentration are small fractions of the total concentration in the manifold and are transmitted more quickly and efficiently and with less distortion and delay through the manifold. Data Reduction Algorithm The ion intensities for m/z of the molecular ion from the naturally abundant compound and for m/z of the molecular ion from the isotopically enriched standard of the same compound are monitored. These ion intensities are used to determine the concentration of the compound in ambient air. For the typical operating case where the mass filter is tuned to pass only the ion of interest, the general expres sion for the signal intensity of this ion, /, is (2-3)

h ' c . ^ V i i

+

C

*ZW*

(«>

The terms C, and C are the ambient and internal standard analytical concentrations, respectively. The Κ terms are the fractional abundances of the various isotopomers of the analyte. They are calculated from a knowledge of the isotopic abundances of the analyte in ambient air and the standard. The Ρμ terms are the fragmentation factors for the parent molecules present in ambient air which can contribute to the intensity at m/z,. The F^ terms are the fragmentation factors for the parent molecules in the isotopically labeled standard gas added to the ambient sample. These terms are determined from mass spectrometric studies of the fragmentation of the analyte under the ionization conditions used in the analysis. Algorithms for S 0 (4) DMS (3), OCS (5) and C S (2) have been developed. Procedures for modifying the algorithm for small changes in ionization conditions are described in these references. For DMS (3) and C S (2) the standard isotopomers monitored, DMS-c^ and C S , are not present in the ambient air and the ambient isotopomers monitored, DMS and C S , not present in the standard at measurable levels. Consequently, the atmospheric and the standard concentrations are proportional, making 8

2

y

2

2

M

2

32

2


25.

BANDY ET AL.

413

Key Sulfur-Containing Compounds 32

calibrations and calculations simple. Because the S content of our older OCS and S 0 standards was a few percent, typically 4.5%, the calibration curve had an intercept because the ambient and standard isotopomers are present in both the standard and ambient air. Recently OCS and S 0 standards were prepared with sulfur containing less than 0.01% S . These standards make the ratio of ambient and standard concentrations for OCS and S 0 strictly proportional to R at R-values below 0.2. This greatly improves the precision of the measurement of these species at low concentration. 2

2

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Atmospheric Measurements The concentrations of sulfur gases in a relatively unpolluted atmosphere are below 1 part per billion by volume (ppbv). Carbonyl sulfide, OCS, is the most abundant species, having an average concentration of about 500 pptv (6). Because the OCS atmospheric lifetime is more than one year, its fluctuations are small. The analytical challenge, therefore, is to determine the small variations in its concentration with high precision. Short term fluctuations are less than 5% except in polluted air masses. The precision afforded by GC/MS/ELS makes such measurements possible. Sulfur dioxide and dimethyl sulfide are present in the unpolluted atmosphere at concentrations below 50 to 200 pptv. In many areas both species are present below 20 pptv. The lower limit of detection must be about 1 pptv for determining these species. Because the atmospheric lifetimes of DMS and S 0 are short, hours to a few days, significant and informative fluctuations occur on time scales of a few minutes. To capture this information sampling times must be comparably short. Carbon disulfide is present in the background atmosphere at 0.2 to 10 pptv. Its low concentration and short lifetime require a lower limit of detection below 0.2 pptv and sampling times and frequencies of a few minutes or less. Because C S is ubiquitous in nonpristine atmospheres, contamination is a common problem. Otherwise its unreactivity makes sampling simple. 2

2

Sampling System. Because of the generally high reactivity of some of the atmospheric sulfur gases and their low concentration, special attention is needed to reduce sampling losses and contamination. The manifold developed for collecting and analyzing samples is shown in Figure 1. We use this manifold for collecting and analyzing air samples in real time and for collecting grab samples. These grab samples are refrigerated with liquid nitrogen and returned to the laboratory for analysis. The main manifold, constructed from perfluorinated ethylene-propylene Teflon tubing, transports ambient air into the aircraft or laboratory and then to a pump located near the instrument. The pump is constructed from stainless steel and Teflon and is a bellows type (Metal Bellows Corp.). Operating the manifold after the pump at about 10 psi reduces contamination from leaks of ambient air into the manifold. The manifold air flow rate, maintained at about 15 L min* , is monitored by a mass flow meter. The manifold air is exhausted through a needle valve, which is used to control the manifold pressure. 1


414


The main manifold flow is sampled by a secondary Teflon manifold at a flow rate of 200 to 1200 mL min* . This air is dried by a Nation dryer and passed though an unpacked Teflon trap cooled by liquid argon. The trap removes the sulfur gases from the air stream. A mass flow meter is used to monitor the trap flow rate from which the total volume of air sampled is computed. Air passing through the mass flow meter and trap pump is returned to the main manifold through tubing coaxial with the Nafion dryer. Because much of the water has been removed by the cryogenic trap and the pressure is lower because of the pressure drop across the sampling valves, the water mixing ratio in this air is much lower that the incoming ambient air. This difference in mixing ratio provides the driving force for drying the incoming air stream in the Nafion drier. Pretraps to remove ozone and other oxidants are required for the determination of DMS, DMSO and D M S 0 . Normally the trap contents are volatilized by hot water and analyzed by a GC/MS attached directly to the sampling system. In the grab sampling mode the trap is removed, stored under liquid nitrogen and returned to the laboratory where the contents also are volatilized using hot water and analyzed by GC/MS. During storage the grab samples are maintained at liquid nitrogen temperatures. Calibrations are carried out by standard addition of the isotopically labeled analyte to ambient air at a point very near the inlet of the manifold using another Teflon manifold. Thus, losses in the main manifold or instrument affect the standard and ambient analyte proportionately, thereby having no affect on the accuracy of the method unless they are so large that the detection limit is ap proached. Since the MS can separately and simultaneously monitor the labeled (standard) and unlabeled (atmospheric) analyte, a standard addition calibration is included in every sample, thereby eliminating the need for a separate calibration sequence. Using this approach, very precise determinations of the analyte can be made. When sampling speed is not critical the pump can be placed after the point where the manifold is sampled to reduce the risk that the analyte would be removed in the pump and the risk that nonambient analyte may enter the manifold through leaks in the pump. Well maintained pumps do not have these problems, but pump maintenance in the field can be a serious and sometimes unnecessary burden. Usually we pressurize the manifold when aircraft platforms are used to maintain manifold and trap flow rates while at high altitude. Mass chromatograms for S 0 are shown in Figure 2 for a sample obtained south of Barbados in September, 1989. The peak height ratio was 0.064 and the standard concentration was 1576 pptv. Using an algorithm based on the distribution of isotopomers in the sample and standard


1

2

2

( 0 , 9 1 8 J ?

0

Ό

4

2

)

C(ambient SO,) = " Cistandard S0 ) = 2%pptv (5) (0.9457 - 0.0435R) 2

2

2

Measurements of D M S O and D M S 0 . These species are of interest in the atmosphere because they are formed in one channel of the oxidation of DMS that does not lead directly to S 0 (7). Sampling of these species is complicated by the conversion of DMS to DMSO and DMSO to D M S 0 in the trapping phase. We 2

2

2


25.

415

Key Sulfur-Containing Compounds

BANDY ETAL.

AIR INLET FILTER

DRYER


PUMP MASS FLOW CONTROLLER

HELIUM CYLINDER

[MASS FLOW METER GC COLUMN

MASS FLOW ICONTROLLER

AIR

CAL GAS

MASS SPECTROMETER

L-4 LIQUID ARGON TRAP

VENT Figure 1. Pressurized manifold system for sampling ambient sulfur gases. 2211]

ni) P $ Q (Ambient)

2NN

2

m

m m \m \m m m m m m

m m m m

m Ml

M

1

1

1

1

1

1

Ύ

~Γ~ m

1

^Τ^ΦΤ^ΦΊ 1 ' ι · ι 1 · ι ' ι ' ι 1 1 1 ' ι 1 ' ι ' ι κ

M

n

IN

m

iso

m

200 m

1ÎC

Figure 2. Chromatogram of ambient S 0 with ^SC^ added. 2


2H

416


are developing the following GC/MS/ILS scheme for accounting for these conversions as well as losses of these species during sampling and analysis. We use the manifold system shown in Figure 1. A standard containing known amounts of c^-DMSO, clj-DMSOj and (CD ) S is added to the manifold at its entrance. DMS, DMSO and D M S 0 are trapped and brought to the gas chromatograph/mass spectrometer for analysis. The DMS is cryogenically trapped, whereas the DMSO and D M S 0 are trapped on solid adsorbents. The molecular ions monitored are mlz 62, and 70 for DMS, mlz 78, and 84 for DMSO and mlz 94, and 97 for D M S 0 . Conversion of DMS to DMSO and D M S 0 can be accounted for using the peak areas for each of the isotopomers. Mass balance among the isotopomers is shown schematically in Figure 3. The experimentally determined parameters are the peak areas S , S , S , Sg4, S*, and Syj. The standard concentrations, [DM^S-dJ, [DMSO-dJ and [DMS0 -cy, are known. Mass balance considerations lead to the following relationships for unambiguously determining the concentrations of ambient DMS, DMSO and D M S 0 : 34

3

2

2

2

2


2

62

70

78

2

2

[DMS]^ = A )

Λ) an

*70

[DMSO]^ = [ A ) Λ

[DMSO^

- A) Ά

84

= A) Λ

Key Sulfur-Containing Compounds in the Atmosphere and Ocean

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